1. Field of the Invention
The present invention relates to an optical waveguide device, and more particularly, to an optical waveguide device in which an optical waveguide formed on a substrate includes a main waveguide having signal light propagating therein and an undesired light waveguide used to remove undesired light from the main waveguide.
2. Description of Related Art
In the fields of optical communications, optical measurements, and optical information processing, optical waveguide devices have been used in which an optical waveguide is formed on a dielectric substrate of lithium niobate or the like. In the optical waveguide used in the optical waveguide devices, portions branching or combining the optical waveguide are formed, like Mach-Zehnder type Optical waveguides widely used in optical modulators, optical switches, or the like.
In the portion combining the optical waveguides, out-phase light is radiated into the substrate as a radiation mode light, as described in Japanese Laid-open Patent Publication No. 2006-301612. Therefore, in order to cause the radiation-mode light not to be re-coupled to signal light, shielding means is provided or an optical waveguide guiding the radiation-mode light to the outside is provided.
As shown in Japanese Laid-open Patent Publication No. 2008-089875, when higher-mode light is included in signal light at the time of branching the optical waveguide, there is a problem in that that a branching portion of an optical waveguide has a difficulty in branching the signal light at a predetermined branching ratio. Therefore, unnecessary higher-mode light is removed from signal light in front stage of the branching portion. As this removal method, an optical waveguide that guide the higher-mode light to the outside is provided.
On the other hand, as described in Izutsu et al. “Integrated Optical SSB Modulator/Frequency Shifter”, IEEE Journal of Quantum Electronics, vol. QE-17, No. 11, 1981, pp. 2225-2227, an integrated modulator structure is proposed in which plural optical waveguides are integrated. Plural branching portions or combining portions are formed in such an optical waveguide device. Accordingly, undesired light from the optical waveguides arranged along the outside of the overall optical waveguides can be easily removed, but it is very difficult to remove undesired light from branching portions or combining portions arranged inside the overall optical waveguides.
In order to achieve an increase in frequency bandwidth of a modulation signal or a decrease in driving voltage of an optical modulator or the like, a substrate of an optical waveguide device is formed of a thin plate with a thickness of 30 μm or less. In this thin plate, undesired light such as radiation-mode light radiated into the substrate propagates with the substrate itself as a slab waveguide, the probability of re-coupling of the undesired light with signal light increases a lot.